JPS61169822A - Orthogonal polarization type optical frequency shifter - Google Patents

Abstract

PURPOSE:To eliminate the need for a quarter-wavelength plate and to use laser light which has two polarized light components by further separating separated P and S polarized light components of he laser light into diffracted light and 0-order light through an acousto-optic element, and detecting their quantities by photodetectors. CONSTITUTION:The P polarized beam 33 of the laser light 32 is transmitted through a polarization beam splitter 20 and the S polarized light beam 36 is reflected; and the two beams are sent out by acousto-optic elements 22 and 25 as 0-order light beams 35 and 37 and diffracted light beams 36 and 38. The beam splitter 25 transmits the beam 36 as P polarized light and reflects the beam 38 to obtain two light beams 39. The 0-order light beams 35 and 37 are detected by photodetectors 23 and 29 to obtain their quantities and a control circuit 30 controls transducers 22 and 28. Therefore, no quarter-wavelength plate is required and temperature dependency is eliminated; and the incident laser light having P and S polarized light components is usable and two mutually orthogonal polarized light beams is equalized at any time.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention diffracts an incident laser beam on the wavefront of a traveling ultrasound in an acousto-optic element (black cell), and shifts the frequency of the laser beam by the Doppler effect. The present invention relates to a device that generates light having a certain frequency difference with respect to an optical signal to be detected by utilizing the effect.

In particular, regarding a device that converts a laser beam having an optical frequency into a laser beam having two mutually orthogonal linearly polarized components having slightly different optical frequencies, this device is referred to as an orthogonal polarization type optical frequency shifter. Here, one of the two orthogonal linearly polarized components is used as an optical signal to be detected, and the other is used as an optical signal of local oscillation output. Note that in order to detect the optical frequency difference (for example, 1 MHz), for example, the output light beam of the orthogonal polarization type optical frequency shifter is transmitted through a 45° polarizer and then passed through a photodetector.

Recently, high-precision, non-contact optical measurement that takes advantage of the properties of light has attracted attention. A heterodyne detection method is used. This optical heterodyne detection method is similar to radio heterodyne reception, in which a local oscillation output signal is mixed with the signal to be detected, an intermediate wave signal (beat signal) having a difference frequency is generated, and signal processing is performed. . Telecommunications uses completely independent oscillators to obtain local oscillator output signals;
In the case of optical interference measurement, it is difficult to manufacture an independent optical oscillator whose frequency of the intermediate wave signal is stable to the extent that there is no fluctuation. Therefore, a laser beam having a certain frequency difference with respect to the optical signal to be detected is generated, sent to the receiving end through a reference optical path, and used as a local oscillation output signal.

[Conventional technology]

Conventionally, an orthogonal polarization type optical frequency shifter has a configuration as shown in Fig. 2.
) is emitted as linearly polarized light (S-polarized light) having an electric field component perpendicular to the paper surface, and is converted into two light beams by a half mirror 3, namely a transmitted light beam 4 and a reflected light beam 5. separated into The transmitted light beam 4 is
The angle θB (
black angle). This acousto-optic element 6 receives a high frequency signal (for example, a center frequency of 42 MH) from a drive circuit 7.
z), the transducer 8 is excited to propagate the ultrasonic signal into the medium body. Therefore, after the transmitted light beam 4 enters the medium body, it is divided into the zero-order light traveling straight and the ultrasonic wave. The diffracted light 9 is split into diffracted light 9 that is diffracted at an angle θB with respect to the wavefront of the signal, and the diffracted light 9 is used here. This diffracted light 9 has a center frequency of 42HH of the acousto-optic element 6.
Shift the frequency of the optical signal by 2, and the optical frequency is f,)
+42MHz.

Next, this diffracted light 9 is reflected by a mirror 10 and transmitted through a 172-wave plate 11 whose optical axis is rotated by 45° around the traveling direction (optical axis) of the light beam.

This transmitted light beam 12 becomes linearly polarized light (P-polarized light) having an electric field component parallel to the plane of the paper, and is transmitted through the polarizing beam splitter 13.

On the other hand, the reflected light beam 5 from the half mirror 3 acts on the acousto-optic element 14 in the same way as the acousto-optic element 6 described above.
Of the zero-order light and the diffracted light 16 that are incident at an angle θB that meets the black condition and sent out, the latter diffracted light 16 is used. Note that the center frequency of the high-frequency signal supplied from the drive circuit 7 to the transducer 15 is 43 MHz, which is higher by I MHz than the center frequency 42 MHz of the acousto-optic element 6 in this example, so that this diffracted light 16 is acoustic. The frequency of the optical signal is shifted by the center frequency of the optical element 14, 43 MHz, and the optical frequency becomes f6+43HH2.

Next, this diffracted light 16 is reflected by a mirror 17,
The light enters the polarizing beam splitter 13 described above. Since this incident diffracted light 16 is originally S-polarized light, it is reflected by this polarizing beam splitter 13.

In this way, the P-polarized transmitted light beam 12 and the S-polarized diffracted light 16 are transmitted and reflected by the polarized beam splitter 13, so that the emitted light beam of the polarized beam splitter 13 has a light component as a component. Frequency (f
o + 42'HH7) P polarization and optical frequency (t'o
+43HH2) S polarized light is on the same optical path. Two light beams with a mutual frequency difference of I MHz are obtained.

[Problem that the invention seeks to solve]

The conventional orthogonal polarization type optical frequency shifter requires a 172-wave plate 11 to be inserted on the optical path. This 172-wavelength plate is manufactured by polishing a birefringent material such as quartz to a predetermined thickness, but manufacturing is extremely difficult because the thickness accuracy requires submicron order. Furthermore, since birefringence varies depending on temperature, temperature compensation means such as bonding together two quartz plates whose polarities are opposite to each other due to temperature changes in birefringence is required. Therefore, if a 172-wavelength plate were manufactured to be usable for practical use, it would be extremely expensive.

In addition, the conventional orthogonal polarization type optical frequency shifter has its output light 1
When it is desired to equalize the optical intensities of the P-polarized beam and the S-polarized beam, which are the components of 8, the power of the high-frequency signal of the transducer drive circuit 7 of the acousto-optic elements 6 and 14 is adjusted to reduce the diffraction of the acousto-optic elements 6 and 14. By changing the efficiency, the respective intensities of the P-polarized beam and the S-polarized beam of the emitted light 18 were adjusted while being detected by a photodetector. However, if a deviation occurs in the position adjustment of the optical system components due to disturbance such as vibration, and the light intensities of the P-polarized beam and the S-polarized beam differ, the above adjustment will be necessary each time, and the orthogonal polarization It was not possible to use the wave type optical frequency shifter continuously.

A first object of the present invention is to provide an orthogonal polarization type optical frequency shifter without using a 172-wave plate having the above-mentioned problems. The second object of the present invention is to
An object of the present invention is to provide an orthogonal polarization type optical frequency shifter that always equalizes the respective intensities of two mutually orthogonal linearly polarized lights (P polarized light and S polarized light) of an emitted laser light beam. The third object of the present invention is to provide an orthogonal polarization type that can use incident laser beams having each component of P-polarized light and S-polarized light, whereas in conventional products, the incident laser light is limited to S-polarized light. An object of the present invention is to provide an optical frequency shifter.

[Means for solving problems]

The present invention has been made to achieve the above object, and a first aspect of the present invention is to input a laser beam having each component of P-polarized light and S-polarized light, separate it into P-polarized light and S-polarized light, and emit the separated light. an acousto-optic element that receives one of the P-polarized light beams and the S-polarized light beams emitted from the first polarized beam splitter and emits a zero-order light beam and a diffracted light beam; a second polarizing beam splitter or a non-polarizing beam splitter that receives the other beam and the diffracted light beam and outputs a combined light beam of P-polarized light and S-polarized light; and a second polarized beam splitter or non-polarized beam splitter that detects the zero-order light beam and outputs an electrical signal. a photodetector; a control circuit that receives an electrical signal from the photodetector and outputs a control signal according to the electrical signal; and a high-frequency signal that receives a control signal from the control circuit and adds the control signal. A second aspect of the present invention is an orthogonal polarization type optical frequency filter characterized by comprising a drive circuit that supplies the transducer of the acousto-optic element with a drive circuit that supplies the transducer of the acousto-optic element. and a first polarizing beam splitter that separates the beam into P-polarized light and S-polarized light and outputs them, and the P-polarized beam and S-polarized beam emitted from the first polarized beam splitter are incident thereon to form a zero-order light beam and a diffracted light beam. a first acousto-optic element and a second acousto-optic element that each emit a
A second polarizing beam splitter or a non-polarizing beam splitter receives the two diffracted light beams and outputs a combined light beam of P-polarized light and S-polarized light; a photodetector that outputs a signal; a control circuit that receives electrical signals from each of the photodetectors and outputs a control signal according to the difference between the electrical signals; and a control circuit that receives a control signal from the control circuit and outputs the control signal. is applied to the high frequency signal to the first acousto-optic element and the second
This is an orthogonal polarization type optical frequency filter characterized by comprising a drive circuit that supplies power to each transducer of an acousto-optic device.

〔Example〕

FIG. 1 is a configuration diagram showing an embodiment of an orthogonal polarization type optical frequency shifter according to the present invention, and in the same figure, 19 is a He-M
e-gas laser (wavelength: 632.8jm, optical frequency f
, = 474. ITH2), 20 and 25 are the first polarizing beam splitter, and the second polarizing beam splitters 21 and 21 are the first acousto-optic element (center frequency; 80HH2) and the second acousto-optic element (center frequency; 81 MHz), 22 and 28 are transducers 23 and 29 installed in each acousto-optic medium of the first acousto-optic element 21 and the second acousto-optic element 27, respectively.
is a photodiode as a photodetector and its driving circuit, 2
4 and 26 are mirrors, 30 is a control circuit, and 31 is a high-frequency signal (8
0HH2 and 81t4Hz).

Next, the operation of this example will be explained in detail. The laser beam 32 emitted from the He-Me gas laser 19 is a linearly polarized beam having an orientation of 45 degrees with respect to the optical axis on the paper, and this is incident on the first polarizing beam splitter 20 having a polarization separation function. The P-polarized laser beam 33 is transmitted, and the S-polarized laser beam 34 is reflected and separated into two light beams. The first polarizing beam splitter 20 has a cubic overall shape, and is made by joining the bottom surfaces (squares) of two triangular prism-shaped glass substrates to each other via a dielectric multilayer film. Zinc sulfide (refractive index;
A plurality of layers (in this example; 23 Layer) Laminated. Note that a second polarizing beam splitter 25, which will be described later, also has a similar structure. Next, the P-polarized laser beam 33 is incident on the wavefront of the ultrasonic signal of the first acousto-optic element 21 while forming a Black angle θB. This acousto-optic element 21 is made of tellurite glass (manufactured by HOYAII:
It consists of a transducer 22 (electrode formed on the amphipod surface of a piezoelectric plate made of a Limb Os 36°Y plate, resonant frequency: 80 MHz) installed on the side of an acousto-optic medium made of AOT-5), and driven. A high frequency signal (80HH2) is supplied from the circuit 31 to the transducer 22 to excite it, and an ultrasonic signal is propagated within the medium of the acousto-optic element 21. As a result, the incident laser light beam 33 is divided into a zero-order light 35 that travels straight and a diffracted light 36 that is diffracted at an angle θB with respect to the wavefront of the ultrasonic signal and sent out. This diffracted light 36 shifts the frequency of the optical signal by the center frequency 80H1lz of the acousto-optic element 21,
The optical frequency is f1+80HH2. Next, this diffracted light 36 is reflected by the mirror 24, enters a second polarizing beam splitter having a coupling function, and is transmitted as is since it is P-polarized light. The zero-order light 35 enters the photodetector 23 and its light amount is detected.

On the other hand, the S-polarized laser beam 34 is reflected by the mirror 26 and enters the wavefront of the ultrasonic signal of the second acousto-optic element 27 at a Black angle θB. This second acousto-optic element 27 has basically the same structure as the first acousto-optic element 21, and is equipped with a transducer 28 on the side of the medium.
A high frequency signal (81HH2') is supplied from the drive circuit 31 to this. Here again, the incident laser beam 3
4 is divided into a zero-order light 31 that travels straight and a diffracted light 38 that is diffracted at an angle θB with respect to the wavefront of the ultrasonic signal.

The zero-order light 37 enters the photodetector 29, and the amount of light is detected. The diffracted light 38 shifts the optical signal by the center frequency 818112 of the second acousto-optic element 27, and the optical frequency is f
1+81MHz. This diffracted light 38 also enters the second polarization beam splitter 25 described above, and since it is S-polarized light, it is reflected and sent out. At this time, since the second polarization beam splitter 25 has a function of combining P-polarized light and S-polarized light, the component of the emitted light beam 39 is the P-polarized light (light frequency: f, +80 MHz) of the above-mentioned diffracted light 36.
) and S-polarized light of diffracted light 38 (light frequency: fl+81MH
) on the same optical path, with a mutual frequency difference of 1MHz
Two light beams are obtained.

Next, the aforementioned O-order lights 35 and 36 are detected by photodetectors 23 and 29, respectively, and converted into electrical signals. Then, each electric signal (voltage) is supplied to a control circuit 30 containing a differential amplifier, which outputs a control signal according to the voltage difference.

This control signal is added to the high frequency signal of the transducer 22.28 of the drive circuit 31 described above and is C1 modulated.

Generally, the relationship between the high frequency signal applied to the transgy 1-1 of the acousto-optic element and the respective optical intensities of the 0th order light and the 1st order diffracted light is as follows when the power of the high frequency signal is increased to within a range of zero. The light intensity of the 0th-order light monotonically decreases, the light intensity of the 1st-order diffracted light monotonically increases, and the light intensity curves intersect each other at intermediate powers. In the present invention, the P-polarized zero-order light beam 35 emitted from the first acousto-optic element and the S-polarized zero-order light beam 31 emitted from the second acousto-optic element are used as monitors for the photodetectors 23 and 23, respectively. 29
Detect each light intensity as an electrical signal (voltage)
Then, the electric signal difference (voltage difference) is controlled to be zero, that is, the light intensity of the 0th-order light beams 35 and 37 is equalized, and the P-polarized light beam 36 and the S
The respective light intensities of the polarized beams 38 are made equal to each other. For example, when the light intensity of the 0th order light beam 35 (P polarized light) increases relative to that of the 0th order light beam 31 (S polarized light), the first
The power of the high-frequency signal sent to the transducer 22 of the acousto-optic element 21 is increased by the amount of the control signal, and the zero-order light beam 35 is
(P-polarized light) (as a result, the light intensity of the first-order diffracted light beam 36 (P-polarized light) is increased), and the zero-order light beam 35 (P-polarized light) and the zero-order light beam 31 (S-polarized light) are (As a result, the light intensities of the first-order diffracted light beams 36 (P-polarized light) and 38 (S-polarized light) are made equal.). Conversely, when the light intensity of the 0th order light beam 37 (S polarized light) increases relative to that of the 0th order light beam 35 (P polarized light), the transducer 2 of the second acousto-optic element 21
By increasing the power of the high frequency signal to 8 by the amount of the control signal, it becomes 0.
The light intensity of the first-order light beam 37 (S-polarized light) is decreased (as a result, the light intensity of the first-order diffracted light beam 38 (S-polarized light) is increased), and the zero-order light beam 35 (P-polarized light) and the zero-order light beam 31 are
(S-polarized) light intensities are made equal to each other. The above example is a means to decrease the light intensity of one zero-order light when the light intensity of one zero-order light increases, but the other zero-order light
Means for increasing the light intensity of the secondary light are also effective. For example, when the optical intensity of the 0th order light beam 35 (P polarized light) increases relative to that of the 0th order light beam 37 (S polarized light), the high frequency signal power to the transducer 28 of the second acousto-optic element 27 is divided into the IIJWJ signal. The light intensity of the zero-order light beam 37 (S-polarized light) may be increased by decreasing the amount of light.

As a result, the transmitted light beam 39 emitted from the orthogonal polarization type optical frequency shifter has its component P polarized light (optical frequency;
f , +818H2) and S-polarized light (light frequency: f +
+818H2) can always be controlled equally.

In addition to the embodiments described above, the present invention provides a method in which each center frequency of the first and second acousto-optic systems is arbitrarily selected relative to the frequency of a desired intermediate wave signal (e.g. 0.IHH2). ; 60H1IZ and 60.1H11Z). Since the frequency of the intermediate wave signal and the center frequency difference between the first and second acousto-optic elements are equal, the center frequency of the acousto-optic element may be selected depending on the individual purpose. In the embodiment, two acousto-optic elements 21 and 25 are used, but only one of them may be used and the center frequency of the acousto-optic element may be set to the frequency of the intermediate wave signal. For example, when only the first acousto-optic element 21 is used, each beam of P-polarized diffracted light 36 and S-polarized light 34 enters the second polarization beam splitter 25, and the P-polarized zero-order light beam 35 is used as a monitor. is detected. In this case as well, the light intensity of the zero-order light beam 35 is converted into an electrical signal by the photodetector 23, and a high-frequency signal to which a control signal corresponding to the electrical signal is added is supplied to the transducer of the first acousto-optic element. control. For example, control when the light intensity of the diffracted light beam 36 (P-polarized light) increases relative to that of the S-polarized light beam 34 is performed by controlling the first acousto-optic element 21 as described in the above embodiment.
The high-frequency signal power to the transducer 22 is reduced by the amount of the control signal, the light intensity of this zero-order light beam 35 is increased, and the light intensity of the diffracted light 36 (P-polarized light) is decreased,
The light intensity may be made equal to the light intensity of the S-polarized beam 40. 1st
The second polarizing beam splitter may be constructed of a Wollaston prism, an O-John prism, etc., and the second polarizing beam splitter may maintain a substantially constant polarization state if optical loss is acceptable. A non-polarizing beam splitter that emits light may also be used. This non-polarizing beam splitter has a cubic overall shape, with a multilayer film described later being deposited on the bottom surface (square) of one triangular prism-shaped glass substrate, and the bottom surface (square) of the other triangular prism-shaped glass substrate. This multilayer film has equal energy transmittance for P-polarized light and S-polarized light, and equal energy reflectance for P-polarized light and S-polarized light, regardless of the polarization state of the incident light. Example: energy transmittance: 47%, energy reflectance: 46%), specifically a high refractive index material layer (H layer) such as zinc sulfide (refractive index: 2.29),
It is formed by sequentially stacking a low refractive index material layer (L layer) such as cryolite (refractive index: 1.25), the H layer, the silver thin film layer, the H layer, the above layer, and the H layer. . As for the laser light, in addition to a gas laser, a semiconductor laser, a dye laser, a solid-state laser, etc. having each component of P-polarized light and S-polarized light may be used.

〔Effect of the invention〕

As described above, according to the present invention, the 172-wavelength plate, which was an essential optical component of conventional products, is no longer necessary, so temperature dependence is eliminated, and a relatively inexpensive polarizing beam splitter and mirror are used. Thus, an orthogonal polarization type optical frequency shifter can be realized. Moreover, the P-polarized light of the emitted light and each component at the SWA destination can always be made equal. Furthermore, since the components of the incident laser beam need only have at least each of P-polarized light and S-polarized light (the light intensity of each component does not matter), the field of use can be expanded.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing an embodiment of an orthogonal polarization type optical frequency shifter according to the present invention, and FIG. 2 is a configuration diagram of a conventional orthogonal polarization type optical frequency shifter. 19... Laser light, 20... First polarizing beam splitter, 21... First acousto-optic element, 22.28...
Transducer, 23.29... Photodetector, 25.
...Second polarizing beam splitter, 21...Second acousto-optic element, 30...Control circuit, 31...Drive circuit

Claims (2)

[Claims] (1) A first polarizing beam splitter that inputs a laser beam having each component of P-polarized light and S-polarized light, separates it into P-polarized light and S-polarized light, and outputs it; An acousto-optic element that receives one of the S-polarized beams and outputs a zero-order light beam and a diffracted light beam, and an acousto-optic element that receives one of the S-polarized beams and outputs a zero-order light beam and a diffracted light beam; a second polarizing beam splitter or a non-polarizing beam splitter that emits a beam, a photodetector that detects the zero-order light beam and outputs an electrical signal, and receives an electrical signal from the photodetector,
a control circuit that outputs a control signal according to the electrical signal;
An orthogonal polarization type optical frequency shifter comprising a drive circuit that receives a control signal from the control circuit and supplies a high-frequency signal to which the control signal has been added to a transducer of the acousto-optic element. (2) A first polarizing beam splitter that inputs a laser beam having each component of P-polarized light and S-polarized light, separates it into P-polarized light and S-polarized light, and emits it, and a P-polarized beam that is emitted from the first polarized beam splitter. and a first acousto-optic element and a second acousto-optic element which respectively receive the S-polarized light beam and output a zero-order light beam and a diffracted light beam;
A second polarizing beam splitter or a non-polarizing beam splitter receives the diffracted light beam of the book and outputs a combined light beam of P-polarized light and S-polarized light, and detects the two zero-order light beams and generates an electric signal, respectively. a control circuit that receives electrical signals from each of the photodetectors and outputs a control signal according to the difference between the electrical signals; and a control circuit that receives a control signal from the control circuit and outputs a control signal. An orthogonal polarization type optical frequency shifter comprising a drive circuit that supplies the applied high-frequency signal to each transducer of the first acousto-optic element and the second acousto-optic element.